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https://issues.apache.org/jira/browse/HADOOP-3387?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12597021#action_12597021
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Subramaniam Krishnan commented on HADOOP-3387:
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The MultiFileInputFormat has the following deficiencies:
1) Assume you have 5 small(100KB) files & the number of Maps specified is 10, 5
Maps will still be allocated when 1 would have been sufficient. This is the
case whenever you have small files(size less than DFS Block Size) & number of
Maps specified is more than the number of files.
2) The MultiFileInputFormat doesn't handle large files efficiently. Assume the
opposite scenario - you have 5 large(~1GB) file & the number of Maps specified
is 3, only 3 Maps will be allocated which is insufficient. This is the case
whenever you have large files(size more than DFS Block Size) & number of Maps
specified is less than the number of files.
3) The MultiFileInputFormat also doesn't handle a mixed bag of large & small
files efficiently as explained above.
The Custom Splitter is a Balanced Multi File Splitter in the sense that it
tries to align the splits closest to the DFS Block Size even if the files are
small, large or a mixed bag.
We have an implementation for text/sequence record readers for our Balanced
Multi File Splitter.
We also have a additional functionality that allows splitting based on user
specified split size(defaulted to DFS Block Size, of course) rather than on
number of splits.
> Custom Splitter for handling many small files
> ---------------------------------------------
>
> Key: HADOOP-3387
> URL: https://issues.apache.org/jira/browse/HADOOP-3387
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Reporter: Subramaniam Krishnan
> Assignee: Subramaniam Krishnan
> Fix For: 0.18.0
>
>
> Hadoop by default allocates a Map to a file irrespective of size. This is not
> optimal if you have a large number of small files, for e.g:- If you 2000
> 100KB files, 2000 Maps will be allocated for the job.
> The Custom Multi File Splitter collapses all the small files to a single
> split till the DFS Block Size is hit.
> It also take care of handling big files by splitting them on Block Size and
> adding up all the reminders(if any) to a further splits of Block Size.
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